1. Field of the Invention
The present invention relates to a fundus camera to take an image of a fundus of a subject's eye for use, for example, in ophthalmic hospitals and mass medical examination.
2. Description of the Related Art
It is well known that to facilitate focusing on a fundus of a subject's eye, images of indexes are projected to the fundus, and a positional relation of index images is observed through a focusing lens of an observing and photographing system, to thereby obtain a clear image of the fundus.
Japanese Patent Application Laid-Open No. 5-95907 discusses a fundus camera, in which two focus split index images projected onto the fundus are captured and a focus state is detected based on positions of the focus split index images while attenuating the brightness of the index images.
Japanese Patent Application Laid-Open No. 8-275921 discusses an ophthalmic apparatus, in which focus index images are projected onto the fundus, the focus index images are captured by a photographing optical system, and a focus state is detected.
Japanese Patent Application Laid-Open No. 1-178237 discloses a modified embodiment of an apparatus for automatic focusing (AF) by capturing an electronic image during observation and detecting contrast between captured images. In other words, when first and second ranges are brought into focus by using high frequency components of fundus images to perform focusing, and a distance in a light axis direction is obtained from the focusing lens positions.
However, a conventional fundus camera divides incident light into a fundus illumination light flux, a focus split index light flux, and an observation photographing light flux in regions near the pupil of the examined eye in order to remove reflected light from the cornea. Therefore, when there are personal differences in aberration in the examined eye's optical system, if an image is captured only based on the focus split index image positions which are predetermined, there is a possibility that errors may occur in focusing for some examined eyes, resulting in a blurred fundus image.
As a solution to this problem, an apparatus is known, in which electronic image sensing is performed even during observation, and automatic focusing (AF) is carried out by detecting contrast among captured images.
In an apparatus as described above, the drawback that a focusing error may occur for some examined eyes, which will result in an out-of-focus image can be solved. However, because regions where a focus is detected are fixed in some portions of the imaging system, there is another problem yet to be solved.
In a conventional method of AF detection, in which, as to some regions of the fundus, fundus images are formed at different distances in the depth direction; therefore, the focus detection ranges have to be fixed, it is necessary to guide the line of sight of the examined eye in such a manner that the regions to be focused may match a focus detection range.
Like general AF single-lens reflex cameras, even if the focus detection range is movable, it must still be moved manually, and further the AF detection position changes with the movement of the eyeball.